Clive Exton in partnership with producer Brian Eastman adapted the pilot. Together, they wrote and produced the first eight series. Exton and Eastman left Poirot after 2001, when they began work on Rosemary & Thyme. Michele Buck and Damien Timmer, who both went on to form Mammoth Screen, were behind the revamping of the series.[3] The episodes aired from 2003 featured a radical shift in tone from the previous series. The humour of the earlier series was downplayed with each episode being presented as serious drama, and an undercurrent of postmodernism saw the introduction of gritty elements not present in the Christie stories being adapted. Recurrent motifs in the additions included drug use, sex, abortion, homosexuality, and a tendency toward more visceral imagery. Story changes were often made to present female characters in a more sympathetic or heroic light, at odds with Christie's characteristic gender neutrality. The visual style of later episodes was correspondingly different: particularly, an overall darker tone; and austere modernist or Art Deco locations and decor, widely used earlier in the series, being largely dropped in favour of more lavish settings (epitomised by the re-imagining of Poirot's home as a larger, more lavish apartment).[4] The series logo was redesigned (the full opening title sequence had not been used since series 6 in 1996), and the main theme motif, though used often, was usually featured subtly and in sombre arrangements; this has been described as a consequence of the novels adapted being darker and more psychologically driven.[5] However, a more upbeat string arrangement of the theme music is used for the end credits of Hallowe'en Party, The Clocks and Dead Man's Folly.

Florin Court in Charterhouse Square, London, was used as Poirot's fictional London residence, Whitehaven Mansions.[11] The final episode to be filmed was Dead Man's Folly in June 2013 on the Greenway Estate (which was Agatha Christie's home) broadcast on 30 October 2013.[12]

Suchet was recommended for the part by Christie's family, who had seen him appear as Blott in the TV adaptation of Tom Sharpe's Blott on the Landscape.[13] Suchet said that he prepared for the part by reading all the Poirot novels and every short story, and copying out every piece of description about the character.[14][15][16] Suchet told Strand Magazine: "What I did was, I had my file on one side of me and a pile of stories on the other side and day after day, week after week, I ploughed through most of Agatha Christie's novels about Hercule Poirot and wrote down characteristics until I had a file full of documentation of the character. And then it was my business not only to know what he was like, but to gradually become him. I had to become him before we started shooting."[17] During the filming of the first series, Suchet almost left the production during an argument with a director, insisting that Poirot's odd mannerisms (in this case, putting a handkerchief down before sitting on a park bench) be featured.[18]

According to many critics and enthusiasts, Suchet's characterisation is considered to be the most accurate interpretation of all the actors who have played Poirot, and the closest to the character in the books.[19] In 2013, Suchet revealed that Christie's daughter Rosalind Hicks had told him she was sure Christie would have approved of his performance.[20]

In 2007, Suchet spoke of his desire to film the remaining stories in the canon and hoped to achieve this before his 65th birthday in May 2011.[21] Despite speculation of cancellation early in 2011, it was announced on 14 November 2011 that the remaining books would be adapted into a thirteenth series to be filmed in 2012.[22] The remaining books were finally adapted in 2013 into 5 episodes, from which, Curtain, was aired last in 13 November 2013. A 2013 television special, "Being Poirot", centered on Suchet's characterisation and his emotional final episode.

Agatha Christie's grandson Mathew has commented, "Personally, I regret very much that she (Agatha Christie) never saw David Suchet. I think that visually he is much the most convincing and perhaps he manages to convey to the viewer just enough of the irritation that we always associate with the perfectionist, to be convincing!"[27]

More recently, the series has been described by some critics as going "off piste",[28] though not negatively, from its old format. It has been praised for its new writers, more lavish productions and a greater emphasis on the darker psychology of the novels. Significantly, it was noted for Five Little Pigs (adapted by Kevin Elyot) bringing out a homosexual subtext of the novel.[5] Nominations for twenty BAFTAs were received between 1989 and 1991 for series 1–3.[29]

In Region 1, Acorn Media has the rights to series 1–6 and 11–12. Series 7–10 are distributed by A&E, a co-producers on several of them. In North America, series 1–11 are available on Netflix and Amazon Prime Instant Streaming service. In Region 4, Acorn Media (distributed by Reel DVD) has begun releasing the series on DVD in Australia in complete season sets. To date, they have released the first 8 series of the show.[31] Series 1–9 and 12 are available in Spain (Region 2) on Blu-ray with Spanish and English audio tracks. Dutch FilmWorks were reported to be the first company to release series 12, in 2010.

Beginning in 2011, Acorn began issuing the series on Blu-ray discs. As of 4 November 2014, series 1 through 13 have all been issued on DVD and Blu-ray by Acorn. The A&E DVD releases of series 7 through 10 corresponded to the A&E versions broadcast in America which were missing sections of the original video as originally broadcast in the United Kingdom. The Acorn releases of series 7 through 10 restore the missing video.

Being Poirot is a 50-minute television programme in which David Suchet attempts to unravel the mysterious appeal of Hercule Poirot and how he portrayed him. It was broadcast in the United Kingdom on the same evening as the final episode Curtain.

Suchet visited Greenway, Agatha Christie's summer home, recollecting how he met her daughter Rosalind and her husband Anthony Hicks for their approval before he began filming. He now meets Christie's grandson Matthew Pritchard who recounts how his grandmother found the character amongst Belgian refugees in Torquay. A visit to the permanent Poirot exhibition at Torquay Museum to which he presented the cane he used in the television series.

Suchet acknowledged the first stage and film adaptations of the books with actors such as Charles Laughton on the London stage in Alibi, an adaptation of The Murder of Roger Ackroyd, in 1928. Alibi was filmed in 1931 with Austin Trevor but is now lost. The oldest surviving film portrayal from 1934 was Lord Edgware Dies again with Austin Trevor portraying Poirot. Suchet notes a conscious decision was made by the film company to portray Poirot without a moustache. Films featuring Albert Finney and Peter Ustinov were also featured and Suchet reveals he read the books and wrote down 93 notes of the character that he would use in his portrayal and for him it was discovering the voice he would use and the rapid mincing gait featured in the books.

Suchet also goes to Florin Court, a place that the production company found to represent his home Whitehaven Mansions. There he meets first producer Brian Eastman, with whom he discusses the set that was built based on the flats and Eastman's decision to fix the stories in 1936. Suchet also visits composer Christopher Gunning who had composed four themes for Eastman, the first being Gunning's favourite. Eastman chose the fourth after having Gunning darken the tone.

Suchet travels to Brussels, where he is feted by the police chief and mayor. He then goes to Ellezelles which claims to be the birthplace of Poirot and is shown a birth certificate as proof. It says the date was 1 April, "fools day" (no year mentioned). Finally, Suchet travels on the Orient Express and recounts filming the episode "Dead Man's Folly" last at Greenway to finish on a high note.